Process and apparatus for extracting zinc

ABSTRACT

A process and apparatus for extraction of zinc from a material containing one or more iron oxides and zinc oxide which facilitate heating a composite body of the material and a carbonaceous material in a reduction zone. The reduction zone contains a reducing atmosphere at a temperature insufficient to effect melting of the iron in the material but at a temperature and for a time sufficient to form a reductant from the carbonaceous material and to reduce a predetermined amount of the zinc oxide to zinc vapor. The process and apparatus also facilitate collecting the zinc vapor and cooling it to form liquefied or solid zinc.

FIELD OF THE INVENTION

This invention relates to a process for the extraction of zinc from azinc containing material, and to an apparatus for carrying out theprocess. The invention is particularly suitable for the extraction ofzinc from material containing one or more iron oxides and zinc oxide,for example electric arc furnace (EAF) dust. The following discussionwill focus on extraction of zinc from EAF dust, however it is to beclearly understood that the invention is not limited to such anapplication. In the application of the invention to EAF dust, iron andlead are also preferably extracted.

BACKGROUND ART

Electric arc furnace (EAF) technology is becoming an increasingly moreimportant means for steel production world-wide. EAF technology,however, suffers from the disadvantage of producing, as a by-product,large quantities of EAF dust. Approximately 15 to 20 kg of EAF dust isformed per tonne of steel, meaning that millions of tonnes of EAF dustare produced annually world-wide. It is considered a toxic waste and itssafe disposal is accordingly problematic. In some jurisdictions,landfill or like disposal of EAF dust is prohibited and so there is asignificant incentive to process the dust into components able to berecycled or otherwise safely disposed of. Moreover, given that EAF dustincludes as major components iron and zinc, and as lesser componentslead and other elements of economic significance, it is a potentiallyvaluable resource yet to be adequately commercially exploited. Zinc inparticular may be usefully recycled to a variety of uses depending onthe purity of the grade able to be extracted.

European patent publication 174641 is directed to the recovery of zincand iron from EAF dusts. The disclosed process involves pelletising thedust with coke as a solid carbonaceous material, preheating the pelletsin a shaft furnace, and thereafter transferring the preheated pellets,with additional reductant, to an induction furnace in which the pelletsare melted. Zinc and lead are recovered as vapour from the inductionfurnace and condensed to crude zinc and lead metal, while pig iron andlead are separated out as molten phases in the induction furnace.

European patent publication 745692 is also concerned with zinc recoverybut from a wide variety of dusts including EAF dust. The essential focusis again reduction, vaporisation, and condensation of the zinc or lead,but here there is no pelletisation or separate preheating stage. Thetreatment furnace is under a substantial vacuum, and the treatment is ataround 750° C., i.e. well below the iron melting temperature butsufficient to vaporise the zinc.

In the process disclosed in international patent publication WO 91/09977zinc vapour is recovered from a melt furnace in which EAF dust is meltedin a proportional mixture of the dust, coal as a carbonaceous materialreductant, and a slag forming flux agent. The zinc vapour is condensedto form zinc metal.

Australian patent 703821 discloses a process for reducing metal oxidefines and producing metal therefrom in which the fines are pelletised ina composite with a carbonaceous material such as brown coal or peat,preheated in a reducing atmosphere to reduce the metal oxide, and then,in a separate chamber, treating the reduced material to produce a moltenmetal containing phase. Retort apparatus suitable for carrying out thisprocess is disclosed in international patent publication WO 01/38455.

There is clearly a need for an effective and safe means for dealing withEAF dust which also realizes at least a portion of its potentiallyextractable value.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process forextraction of zinc from a material containing one or more iron oxidesand zinc oxide, said process including:

-   -   (a) heating a composite body of said material and a carbonaceous        material in a reduction zone containing a reducing atmosphere at        a temperature insufficient to effect melting of the iron in the        material but at a temperature and for a time sufficient to form        a reductant from said carbonaceous material and to reduce a        predetermined amount of the zinc oxide to zinc vapour; and    -   (b) collecting said zinc vapour and cooling it to form liquefied        or solid zinc.

Preferably, the process further includes:

-   -   (c) controlling said zinc vapour to substantially prevent or        minimise its premature recondensation.

Preferably, the process includes:

-   -   (d) reducing a predetermined amount of said iron oxides;    -   (e) further heating the reduced material from which zinc vapour        has been collected to a temperature sufficient to effect melting        of the iron therein; and    -   (f) recovering and cooling the molten iron.

Waste heat and/or heated gases from said further heating are preferablyutilised in formation of the zinc vapour.

In an advantageous arrangement, the controlling of the zinc vapour tosubstantially prevent or minimise its premature recondensation includesproviding gas flow in said reduction zone arranged to drive said zincvapour away from said reduction zone. Preferably, the gas flow in saidreduction zone is generally downwardly and said zinc vapour is driven bysaid gas flow laterally from the reduction zone.

The respective heating steps (a) and (e) are preferably carried out inseparate heating chambers.

In an advantageous application of the invention, the material containingone or more iron oxides and zinc oxide is electric arc furnace (EAF)dust, although other zinc containing materials, such as zinc plantresidues, may be treated by the process. As previously mentioned, EAFdust is a waste by-product of steel production via EAF technology andresults from vaporisation of volatile metals, particularly zinc andlead, which are subsequently oxidised and extracted as a fine dust. EAFdust contains largely a mixture of oxides, composite iron oxides andchlorides and may include a number of different elements, chiefly iron,zinc, calcium, silicon, lead, copper, manganese, chromium and cadmium.

While the present invention is primarily concerned with zinc extractionfrom zinc containing iron oxide material, it can also extend toextraction of other metals present in the zinc containing material. Theoxides of those other metals should be reducible by reductants derivedfrom carbonaceous material. Where the zinc containing material is EAFdust, other metals that may also be extracted include iron, lead andmanganese.

The carbonaceous material used in the inventive process preferably has ahigh surface area and small grain size selected for enhanced chemicalreactivity of the reductant.

Preferred carbonaceous materials are finely divided brown coal or peatbecause of their cellular structure resulting in high chemicalreactivity. Using low rank coal or peat instead of higher rank coal isadvantageous economically, due to the lower cost of low rank coal orpeat. Brown coal or peat also have desirable physical properties, inparticular their ability to plasticise under mechanical shear whichenables the production of cohesive pellets formed by extrusion. Withoutwishing to be limited to a particular mechanism of formation, it isbelieved that shearing causes attritioning of the brown coal or peatparticles resulting in conversion of finely dispersed water in thestructure to a bulk liquid phase, and polymerisation of phenolic ringspresent in the brown coal causing the coal or peat to become wet andplastic. Shearing also produces large numbers and areas of freshlycleaved coal surfaces, thereby maximising the physical association ofcoal/peat particles with metal oxide particles which in turn enhancesthe rate of the reduction reaction.

Alternatively, the carbonaceous material may comprise a higher rank coalor some other finely divided active carbon source material, such assawdust. In this case, it may be necessary to add a binder and/or waterto the mixture of zinc oxide containing material and carbonaceousmaterial so as to produce a cohesive mass able to be formed intopellets, such as by extrusion.

Preferably, the process further includes forming said composite body bymixing a carbonaceous material with the material containing one or moreiron oxides and zinc oxide to produce a cohesive mass, and compactingthe cohesive mass to produce the composite body.

Whatever the type of carbonaceous material, it may be necessary to addadditional water to the mixture so as to produce the desired consistencyof the cohesive mass.

Preferably, the composite body is in the form of a pellet. Morepreferably, the pellet is formed by extrusion of the cohesive mass.

The relative quantities of the carbonaceous material and zinc oxidecontaining material will vary depending on the desired end result. Theamount of carbonaceous material in the composite body is preferably suchthat, when combusted in the heating step or steps, the carbonaceousmaterial provides at least sufficient heat for its carbonisation and forreduction of the zinc and iron oxides and, if recovered, lead oxide, inthe treated material containing one or more iron oxides and zinc oxide.

Typically, heating step (a) of the inventive process is conducted in afurnace chamber, and the reduction reaction preferably largely takesplace at the boiling point of zinc (907° C.).

The vaporisation of zinc enables an effective means of separating zincfrom other elements present in the zinc containing material, especiallywhere that material comprises EAF dust. In the latter case, the boilingpoint of zinc is considerably lower than those of the other metals ofinterest in the EAF dust, especially iron (3000° C.) and lead (1515°).

In step (b) of the inventive process, zinc vapour produced from thereduction reaction is collected and cooled to form, preferably, zincmetal. Typically, the zinc vapour is directed into a condenser in whichthe vapour is cooled to form the zinc metal.

EAF dust and brown coal typically contain refractory materials, such assilica, alumina and calcium and magnesium oxides, as well as sulphur. Toreduce or remove these impurities, the controlled provision of a fluxingagent, or its precursor, is highly desirable, for forming a basic slagin the further heating step (e) and to encourage desulphurizationreactions to occur. If flux is added to the composite, it can be eitheradded separately, preferably in powder form, to the composite body, or,it can be mixed with the zinc and iron oxides containing material andcarbonaceous material that form the composite body, such that thecomposite body subsequently formed is self-fluxing. Alternatively, aflux precursor, such as uncalcined limestone or magnesite, can be addedto the mixture. During reduction of the composite body subsequentlyformed, the flux precursor undergoes calcination to form the flux. Inthe case of uncalcined limestone, it undergoes calcination to form limeduring the reduction step. Adding uncalcined limestone to the compositebody is advantageous because uncalcined limestone is less expensive thanlime, thus making the composite more economical to produce. The flux orflux precursor may comprise a calcium or magnesium oxide or hydroxide,or a carbonate such as limestone (CaCO₃) or magnesite (MgCO₃).

Where the zinc oxide containing material includes other elements apartfrom zinc, the process of the present invention is an effective means ofseparating zinc from those other elements. In the case of EAF dust, theprocess of the invention is able to effectively separate zinc from othermetals, especially iron and lead.

In the embodiment of the invention dealing with EAF dust, the reductantsformed from carbonaceous materials may also reduce other metal oxides,such as iron oxides and lead oxides, in the composite bodies. Suchreduction processes, of the zinc, iron and lead, may proceed inaccordance with the metal reduction processes disclosed in theaforementioned Australian Patent No. 703821, the entire disclosure ofwhich is incorporated herein by reference. AU 703821 deals particularlywith the reduction of iron oxides using reductants formed fromcarbonaceous material present in composite pellets of the carbonaceousmaterial and the iron oxides, and the carbonaceous material providesfuel for melting of the reduced metals, as well as providing reductantfor reducing the metal oxide.

The molten metal phase which may be formed from using EAF dust in thepreferred inventive process including step (e) includes metals otherthan molten iron, such as molten lead which has a relatively lowermelting temperature of 327° C. Moreover, because molten lead has ahigher density than molten iron, molten lead occupies a lower level inthe molten metal bath than molten iron. This feature enables separationof lead from iron, such as by tapping off molten lead via an outletlocated below the lead/iron interface.

The invention also provides apparatus for extraction of zinc from amaterial containing one or more iron oxides and zinc oxides, including:

-   -   a first furnace chamber for receiving composite bodies that        include said material and a carbonaceous material, which furnace        chamber defines a reduction zone in which said composite bodies        may be heated at a temperature insufficient to effect melting of        the iron in the material but at a temperature and for a time        sufficient to form a reductant from said carbonaceous material        and to reduce a predetermined amount of the zinc oxide to zinc        vapour;    -   means to collect said zinc vapour from the first furnace        chamber; and    -   means to receive said collected zinc vapour for cooling the        vapour to form liquefied or solid zinc.

Means is advantageously provided for controlling said zinc vapour tosubstantially prevent or minimise its premature recondensation.

Preferably, the apparatus further includes:

-   -   a second furnace chamber in communication with said first        furnace chamber to receive therefrom reduced material from which        said zinc vapour has been collected;    -   heating means for further heating the received material in the        second furnace chamber to a temperature sufficient to effect        melting of the iron therein; and    -   means to recover and cool the molten iron.

Preferably, the first and second furnace chambers are arranged so thatwaste heat and/or heated gases from said further heating are utilised information of the zinc vapour in the first furnace chamber.

Advantageously, the means for controlling the zinc vapour includes meansassociated with the first furnace chamber whereby the zinc vapour isdriven away from the reduction zone.

The first furnace chamber is preferably heatable to at least atemperature sufficient to convert at least some of the carbonaceousmaterial to a combustible gas and to effect reduction of the zinc oxideto zinc. Also preferably, the chamber includes a heat resistantcontainer for location within the chamber, which container is used forreceiving the composite bodies. In such an arrangement, a gas conduitpreferably extends from the heat resistant container to a gas burner forheating the furnace. Once the heating means has heated the furnace andmixture of reactants to the required temperature, carbon monoxide andother combustible gases are evolved from the reactants and pass throughthe gas conduit to the gas burner. The furnace heating means and gasburner may be one and the same, or different features. Once evolution ofthe combustible gases commences, the gas burner may be lit and theheating means (if a separate feature) turned off. The gas burner canthen be used to maintain the furnace temperature at the desired level.

An outlet for zinc vapour is typically provided laterally in a wall ofthe first furnace chamber so as to comprise or be part of the zincvapour collecting means, and a zinc condenser is advantageously incommunication with the first furnace chamber via the outlet. Zinc vapourproduced during the reduction process passes through the outlet and intothe condenser where it is collected and cooled to form zinc metal.

The condenser may be any suitable container in communication with thefurnace outlet. In operation, the condenser is maintained at atemperature below the boiling point of zinc so that condensation of zincmay occur therein. However, the temperature of the condenser should notbe too cool in order to avoid premature solidification of zinc as finegrained metal, which can easily reoxidise upon exposure to air.Preferably, the condenser is maintained at a temperature in the range400 to 700° C., more preferably between 500 and 600° C. The temperatureis preferably controlled by providing adequate insulation of thecondensation chamber, allowing the latent heat of condensation of zincand the heat in the waste gases to provide the desired heat. A “zincsplash condenser” may be employed and comprises, for example, amotorised rotor that splashes liquid zinc droplets into the path of awaste gas.

Alternatively, the temperature of the condenser may be controlled bymeans of heating means, such as a furnace, preferably a muffle furnacein which the condenser is positioned.

Preferably the condenser is manufactured from a heat and chemicallyresistant material, such as refractory material, preferably a refractoryceramic. A suitable refractory material is fireclay.

In one embodiment the condenser preferably includes a main condenserchamber having an inlet for receiving zinc vapour positioned above thebase of the main condenser chamber such that condensed zinc does notpass back into the furnace chamber. The main condenser chamber maysurround a vapour conduit, the open end of which forms the vapour inlet,extending from the outlet from the reduction zone to a region of thecondenser above its base.

In one embodiment, the zinc condenser is positioned above the furnacechamber and the vapour conduit extends in a substantially verticaldirection from the furnace outlet into an upper region of the condenserchamber. However, preferably the vapour conduit extends in a lateraldirection from the furnace outlet into an upper region of the condenserchamber.

Preferably, the second furnace chamber, which serves as a melting unit,is located vertically below the outlet of the first furnace chamber suchthat the reduced material, e.g. the reduced pellets, is automaticallyand continuously fed into the second furnace chamber under gravity. Themelting unit preferably includes an insulated melting chamber forreceiving the reduced pellets. Once in the melting unit, the temperatureof the reduced pellets is raised sufficiently to effect melting of themetal (and any slag phase, if present). This may be achieved by simplecombustion of the carbonaceous material remaining in the reducedpellets, possibly aided by injection of an oxidising gas through pipesor tuyeres. If necessary, additional fuel may also be added at thistime, for example combustible gases and/or solid materials. Preferably,the oxidising gas is preheated, such as by heat exchange with wastegases from the apparatus, and typically comprises air or some otheroxygen containing gas.

Advantageously, the temperature increase in the second furnace chamberis augmented by employing an external heating means. Preferably, theexternal heating means is an electrical heating means, such as aninduction heater, resistance heater or a submerged arc. An inductionheater is particularly preferred. In the case of an induction heater,the wall of the insulated melting chamber typically accommodates aconductor coil, with the metal to be melted forming the secondary of atransformer. An induction heater can be used alone or in conjunctionwith the oxidising gas assisted combustion.

In one or more embodiments, the apparatus is advantageously based uponthe design of a retort disclosed in the aforementioned internationalpatent publication WO 01/348455, the entire disclosure of which isincorporated herein by reference. In this form, the apparatus includes:

-   -   a thermally insulated casing defining the first furnace chamber        therein;    -   one or more columns provided within the first furnace chamber,        each column comprising a plurality of vertically orientated,        vertically spaced, heat resistant tubes, wherein the        cross-sectional area of each tube is smaller than that of an        adjacent, lower tube, and wherein the ends of adjacent tubes are        arranged so as to provide an annular space therebetween,    -   an inlet through which a combustible charge is fed into the        uppermost tube,    -   an outlet from which reacted charge is removed from the        lowermost tube; and    -   a fluid conduit for conveying combustible volatiles evolved by        heating said charge to a gas burning means for combustion, to        thereby provide heat to the first furnace chamber.

The insulated casing is preferably made of steel, more preferably mildsteel. The tubes themselves are typically made from a suitable heat andchemical resistant material, such as an alloy, which may contain steel.The distance between the columns of tubes and the shell will vary butshould be such as to provide adequate volume for efficient combustion ofthe heating medium used.

The retort may contain two or more laterally spaced columns, eachcomprising a succession of vertically spaced tubes, in order to increasethe throughput of material treated in the retort.

Preferably the or each column includes three or more verticallyorientated, substantially coaxial tubes. These are advantageouslyvertically spaced from each other such that there is partial overlapbetween adjacent ends, thereby defining an annular space therebetween.The annular space enables volatiles evolved during heating and/orreaction of the charge to escape therefrom. After the initial start up,combustible volatiles combust at the annuli, thereby providing heat forsubsequent reaction of the charge, meaning that the external heatingmeans can be turned down or off.

Preferably each vertical tube is suspended at an end thereof within theretort, allowing free discharge of the charge into the adjacent, lowervertical tube. Moreover free suspension of each tube and the absence ofjoins between tubes facilitates thermal expansion and contraction of thetubes and reduces failure due to thermal cycling.

The retort is preferably of a structure that defines two or morecombustion zones. A first combustion zone is typically located in anupper region of the retort and a second combustion zone is typicallylocated in a lower region of the retort. Preferably the first and secondcombustion zones are separated by a wall, and together form a unitarybody. More preferably, the wall supports one of the verticallyorientated tubes which is suspended therefrom. Preferably, the first andsecond combustion zones each accommodate one or more vertical tubes,such that each combustion zone has an annulus between tubes openingtherein. The provision of more than one combustion zone is advantageousin that it enables greater control over the heating process, as will besubsequently discussed in further detail.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a first embodiment of furnaceapparatus suitable for carrying out the process of the invention in abatchwise fashion;

FIG. 2 is a diagrammatic view of a second embodiment of apparatus forcarrying out the process of the invention in a continuous flow fashion;

FIG. 3 is a modification of the molten lead outlet in the apparatusillustrated in FIG. 2; and

FIG. 4 diagrammatically depicts a modification of the second furnacechamber and associated induction heater.

EMBODIMENTS OF THE INVENTION

One embodiment of apparatus used for carrying out the process of theinvention is illustrated in FIG. 1. A first muffle furnace 10 haslocated therein a stainless steel container 12, acting as a retort, thatencloses a furnace chamber in which are placed EAF-brown coal compositepellets 14. A first thermocouple 16 monitors the temperature of thepellets 14 while they are being heated.

The recessed lid 18 of the muffle furnace 10 has an outlet 20 providedcentrally therein and a conduit 22 leading from the outlet 20 away fromthe interior of the muffle furnace 10. Zinc vapour, and other volatilesand gases, are discharged from the first muffle furnace via the outletand conduit. On the underside of the recessed lid 18 is provided anannular ceramic heat shield 23, the purpose of which is to prevent zincfrom condensing on the underside of retort lid 18.

Surrounding the conduit 22 and located within the recessed lid 18 is azinc condenser 24 comprising a condenser chamber 26 and vertical,integral vent pipe 28, leading away from the condenser chamber 26. Thezinc condenser 24 is formed from fireclay, although other refractorymaterials may instead be used.

Surrounding the zinc condenser 24 and located above the first mufflefurnace 10 is a second heating means comprising a second muffle furnace30. A second thermocouple 32 is provided within the second mufflefurnace 30 in order to monitor the interior temperature therein. Thepurpose of the second muffle furnace is to maintain the temperature ofthe zinc condenser 24 in the preferred range of 500-600° C., so as toavoid the premature recondensation of zinc as a fine dust and insteadallow collection of zinc as a bulk liquid phase. Furthermore, the secondmuffle furnace 30 serves to burn any coal volatiles and reduction gasesemitted from the first furnace before their release.

An example of the use of the apparatus of FIG. 1 will now be described.EAF dust as collected in a bag filter was wetted with a small amount ofwater and formed into pellets in a disc pelletiser. Pelletisation of EAFdust, while not necessary to the process of the invention, renders thedust easier to handle and store.

The EAF pellets were then mixed with wet brown coal, in equalquantities, plus additional water (if necessary) and subjected toattritioning and extrusion to produce substantially homogeneous pellets.The extruded EAF-brown coal were then dried, either naturally by placingthem on trays on covered open racks exposed to ambient air, or by forcedrying by application of some heat.

Successive batches of EAF-brown coal pellets 14 were heated in thefurnace chamber of first muffle furnace 10, which defines a reductionzone, at a temperature insufficient to effect melting of the iron in thepellet material but at a temperature and for a time sufficient to form areductant from the brown coal and to reduce a predetermined amount ofthe zinc oxide to zinc vapour. Reduced zinc oxide, in the form ofgaseous zinc, was discharged from the pellet charge at temperatures inexcess of 907° C., and was observed to exit the first muffle furnace 10via the outlet 20 in the recessed lid 18. The zinc vapour travelled viaconduit 22 into the zinc condenser 24. The zinc vapour condensed intozinc metal, shown as reference numeral 34 in FIG. 1, in the bottom ofthe condenser chamber 26. Waste gases were discharged into the interiorof the second muffle furnace via the vent pipe 28.

Upon completion of the reduction of the pellets 14, the condenser 24 wasremoved from the second muffle furnace and cooled. The condenser wasfound to contain a substantial quantity of zinc. The substantially zincfree reduced pellets in the first muffle furnace were transferred to apreheated crucible and a small amount of oxygen was injected. Thiscaused the pellets to melt, and upon cooling they separated into an ironphase and a slag phase.

Samples of the EAF-brown coal pellets, iron slag and zinc phases wereanalysed and the results are presented in Table 1.

There was virtually no zinc in the iron or the slag. The lead in thezinc represented less than 3% of the lead in the feed pellets. Therewere no significant amounts of lead in the iron or the slag.

A second embodiment of the apparatus according to the invention isillustrated in FIG. 2. A retort 150 includes a thermally insulated metalshell or casing 152 defining therein an annular first furnace chamber154. The furnace chamber 154 houses a column 155 of heat resistant tubes156, 158, 160 and 162 which are vertically orientated and verticallyspaced from each other. The cross-sectional area of the tubes 156, 158,160 and 162 increases from the top of the retort 150 to its bottom.Accordingly, the relative cross-sectional area of the tubes is asfollows: tube 156<tube 150<tube 160<tube 162.

The respective ends of adjacent tubes overlap to thereby form openannular spaces 164, 166, 168 therebetween.

The uppermost tube 156 includes an inlet 157 and acts as a feeder tubethrough which the pellet charge material 169 is fed into the retort 150.The charge moves through the tubes 156, 158, 160 and 162 in successionunder the force of gravity. An outlet 170 is provided at the bottom ofthe lowermost tube 162 through which the reacted charge 169 leaves theretort 150.

The furnace chamber 154 defines a reduction zone and is divided intofirst and second combustion zones comprising first and second chambers154 a and 154 b respectively, by a transverse wall 174. The combustionchambers 154 a and 154 b each include a heating means comprising a gasburner 171, 172 respectively which provide an initial heat source forraising the temperature of the charge 169. The gas burner 172 in thecombustion zone 154 b is larger than burner 171 in order to providesufficient heat for commencement of combustion. Furthermore, the gasburner 172 is located near the top of chamber 154 b, whereas gas burner171 is located near the bottom of chamber 154 a.

In the first combustion zone, the temperature of the composite body orpellet charge is raised causing any free water and chemically fixedwater to be evolved from the charge. With increasing temperature, lowtemperature coal volatiles are released (where the carbonaceous materialis brown coal), then carbon dioxide is released from any carbonatebreakdown. Finally high temperature coal volatiles are released.

In the second combustion zone, the temperature of the charge is raisedto a value and for a time sufficient to effect the desired reaction, inthe present case, the reduction of metal oxides to metal.

A fluid conduit 176 extends from the top of chamber 154 a to the top ofchamber 154 b. Gases evolved from heating the charge 169 are evolvedfrom open annuli 166 and 164 and travel through the conduit 176 to thechamber 154 b, where they are combusted. Because combustion in chamber154 b occurs near the top thereof, combustion gas flow will be in agenerally downward direction. Accordingly any zinc vapour produced inchamber 154 b is swept downwards by the combustion gas flow and collectsat the base of chamber 154 b. The zinc vapour then exists chamber 154 bby overflowing into conduit 180, located near the base of chamber 154 b,and travels to a cooling zone in a zinc condenser 124. The combustionconditions in both chambers 154 a and 154 b are operated fuel rich so asto maintain a reducing atmosphere in both chambers thereby minimisingreoxidation of zinc vapour.

The retort 150 further includes a third combustion chamber 178communicating with the second combustion chamber 154 b via the conduit180 and the zinc condenser 124. The third combustion chamber 178 in turncommunicates with a recuperator 182 through which exhaust gases pass toexhaust outlet 184 under operation of an exhaust fan 185. A gas burner183 is provided inside the third combustion chamber 178.

The recuperator 182 includes a conduit 186 through which passesatmospheric air admitted through inlet 189 under action of a fan 193,and an outlet 187 through which preheated air exits. In the recuperator182, the heat from the exhaust gases is transferred to the incoming airto thereby preheat it and the preheated air enters the first and secondchambers 154 a and 154 b via fluid 10 conduits 188 and 190,respectively. Fluid conduit 190 enters chamber 154 b near its top wheregas burner 172 is located. The amount of air flow into the chambers 154a and 154 b can be regulated by dampers 192, 194 respectively.

At the base of retort 150 is provided a metal melting unit comprising aninduction heater 196. The reduced pellets are fed under gravity into theinduction heater 196 via the retort outlet 170. The induction heater 196includes an upper, slag outlet 198 and a lower, molten metal outlet 200.The slag outlet 198 is located at an upper region of the inductionheater 196 and the metal outlet 200 is located at a lower region. Inuse, once the reduced charge is melted in the induction heater 196, anyslag phase that forms on top of the molten metal phase is tapped off viathe slag outlet 198. The molten metal, typically carbon rich iron, istapped off via molten metal outlet 200. Both the slag and metal outlets198, 200 can be closed by removable, moist fireclay plugs.

At the base of the induction heater 196 is a second molten metal outlet202 through which molten lead may be tapped off. Molten lead is denserthan molten iron and therefore tends to accumulate towards the base ofthe induction heater.

A modification of the second molten metal outlet is shown in FIG. 3. Thesecond molten metal outlet 302 feeds to a heatable conduit 304comprising an insulated cast iron pipe 306 having electrical heatingtape 308 wound therearound. The exit 311 of the cast iron pipe 306 ispositioned above the outlet 302 of the induction furnace 296. A leadmould 313 collects the molten lead discharged from the induction heater296.

The operation of the molten metal outlet 302 will now be described.Prior to the first use, the iron pipe 306 is sealed with a plug of solidlead. When a predetermined amount of molten lead has accumulated in thebase of the induction furnace 296 (determined from the mass of pelletsreduced) the heating tape 308 is switched on, thereby melting the leadplug therein and enabling molten lead to flow from the induction heater296 into the heatable conduit 304. Molten lead will flow from the exit311 into the lead mould 313 until the static head in the pipe 306balances the static head of molten metal in the induction heater 296. Inthis manner, the flow of lead can be controlled and discharge of molteniron through the molten metal outlet 302 avoided.

A modification of the second combustion chamber and induction heater atthe base of the retort is illustrated in FIG. 4, in which like referencenumerals preceded by a “4” or “5” relate to the corresponding parts ofFIG. 2, e.g., column 455, retort outlet 470, slag outlet 498 and moltenmetal outlets 500, 502. The base of the second combustion chamber 454 bis funnel-shaped and in effect forms an integral lowermost tube 462 (thefunctional equivalent of lowermost tube 162 of FIG. 2). The top of theinduction heater 496 has also been modified to receive a gas burner 515through an opening therein 517. In addition, further fuel, preheatedair, flux precursor (e.g. limestone) and/or oxygen may be admittedthrough the opening 517, in order to produce sufficient hot waste gasfor heat transfer. Typically, a temperature of at least 1500.degree. C.will be required in the “melting zone” located towards the top of theinduction heater 496, in order to ensure heat transfer sufficient forthe “zinc boiling zone”, towards the base of the second combustionchamber, to be maintained at a temperature of 907.degree. C. or higher.In this way, waste heat and/or heated gases from the melting zone areutilised in the formation of the zinc vapour. Zinc vapour released fromthe zinc boiling zone exits the second combustion chamber 454 b viaconduit 480 and travels to the zinc condenser (not shown).

Without wishing to be limited to a particular reaction mechanism it isbelieved that while some of the zinc produced from the reductionreaction exits the reduction zone as zinc vapour, some may re-condenseto liquid zinc, with the latent heat of condensation being absorbed bythe endothermic reduction reactions. This is clearly problematicalbecause the re-condensed zinc must be boiled off before the temperatureof the reduced pellets can be increased, in order to melt the remainingmetals in the pellets (and any slag phase, if present). This typicallyrequires that the temperature of the “zinc boiling zone”, located abovethe melting zone of the pellet charge, be maintained sufficiently highto provide the latent heat of vaporisation of zinc. This may be providedeither by a separate heating means, or by ensuring that sufficient wastegas flows from the melting zone into the boiling zone, or both.

The required flow of waste gas can be supplied in a number of possibleways, including the following by way of example:

-   1. Combustion of excess carbon in the reduced pellets with air to    form carbon monoxide and nitrogen.-   2. Combustion of solid, liquid or gaseous fuel with air to form    carbon monoxide, water vapour and nitrogen.-   3. Injection of flux precursors (e.g. limestone) into the melting    zone. Carbon dioxide from calcination of the flux precursor then    reacts with excess carbon from the reduced pellets and/or added fuel    to form carbon monoxide.-   4. Injection of inert gas.-   5. A combination of two or more of the above.

It is highly desirable that no free oxygen is allowed to enter the zincboiling zone as reoxidation of zinc can occur.

1. Apparatus for extraction of zinc and lead from a material containingone or more iron oxides, zinc oxide and lead oxide, comprising: a firstfurnace chamber for receiving composite bodies that include saidmaterial and a carbonaceous material, wherein the first furnace chamberdefines a reduction zone in which said composite bodies may be heated ata temperature insufficient to effect melting of the iron in the materialbut at a temperature and for a time sufficient to form a reductant fromsaid carbonaceous material and to reduce a predetermined amount of thezinc oxide to zinc vapour, and reduce predetermined amounts of said ironoxides and lead oxides to iron and lead, respectively; a firstarrangement comprising an outlet from the first furnace chamber which isstructured to collect said zinc vapour from the first furnace chamber byflow of the zinc vapour into the outlet; a second arrangement comprisinga condensing arrangement in communication with said outlet andstructured to receive said collected zinc vapour and to cool the vapourtherein to form liquefied or solid zinc; a second furnace chamber incommunication with said first furnace chamber to receive therefrommaterial remaining after said zinc vapour has been collected from thefirst furnace chamber; a heating arrangement configured to further heatthe remaining material in the second furnace chamber to a temperaturesufficient to effect melting of the iron and lead therein; a thirdarrangement configured to separately recover the molten iron andseparately recover the molten lead therefrom; a thermally insulatedcasing defining said first furnace chamber therein; one or more columnsprovided within said first furnace chamber, each column comprising aplurality of vertically orientated, vertically spaced, heat resistanttubes, wherein the cross-sectional area of each tube is smaller thanthat of an adjacent, lower tube, and wherein the ends of adjacent tubesare arranged so as to provide an annular space therebetween; an inletthrough which a combustible charge is fed into the uppermost tube; anoutlet from which reacted charge is removed from the lowermost tube; anda fluid conduit for conveying combustible volatiles evolved by heatingsaid charge to a gas burning means for combustion, to thereby provideheat to said first furnace chamber.
 2. Apparatus according to claim 1,further comprising a fourth arrangement configured to control said zincvapour to substantially prevent or minimise its prematurerecondensation.
 3. Apparatus according to claim 2, wherein said fourtharrangement includes a further arrangement associated with said firstfurnace chamber whereby said zinc vapour is driven away from saidreduction zone.
 4. Apparatus according to claim 3, wherein said gas flowin said reduction zone is provided generally downwardly and said zincvapour is driven by said gas flow laterally for the reduction zone. 5.Apparatus according to claim 1, wherein said first and second furnacechambers are arranged so that waste heat and/or heated gases from saidfurther heating are utilised in formation of said zinc vapour in thefirst furnace chamber.
 6. Apparatus according to claim 2, furthercomprising: a fifth arrangement configured to tap slag from said secondfurnace chamber.
 7. Apparatus according to claim 1, wherein said thirdarrangement is configured to tap molten lead from said second furnacechamber.
 8. Apparatus according to claim 1, wherein said second furnacechamber is vertically below the first furnace chamber such that saidreduced material is continuously fed into the second furnace chamberunder gravity.
 9. Apparatus according to claim 1, wherein said heatingarrangement is an external electrical heating arrangement.
 10. Apparatusaccording to claim 1, wherein said first furnace chamber defining acooling zone comprises a zinc vapour condenser in communication with anoutlet from said reducing zone comprising a zinc vapour collectingarrangement.
 11. Apparatus according to claim 10, wherein said condenserincludes a zinc splash condenser.
 12. Apparatus according to claim 10,wherein said condenser includes a main condenser chamber having an inletfor receiving zinc vapour positioned above the base of the maincondenser chamber such that condensed zinc does not pass back into thefurnace chamber.
 13. Apparatus according to claim 10, wherein said maincondenser chamber surrounds a vapour conduit, the open end of whichforms the vapour inlet, extending from said outlet from the reductionzone to a region of the condenser above its base.
 14. Apparatusaccording to claim 10, wherein a vapour conduit extends in a lateraldirection from the furnace outlet into an upper region of the condenserchamber.
 15. Apparatus for extraction of zinc from a material containingone or more iron oxides and zinc oxide, comprising: a first furnacechamber for receiving composite bodies that include said material and acarbonaceous material, wherein the first furnace chamber defines areduction zone in which said composite bodies may be heated at atemperature insufficient to effect melting of the iron in the materialbut at a temperature and for a time sufficient to form a reductant fromsaid carbonaceous material and to reduce a predetermined amount of thezinc oxide to zinc vapour; a first arrangement configured to collectsaid zinc vapour from the first furnace chamber; a second arrangementconfigured to receive said collected zinc vapour for cooling the vapourto form liquefied or solid zinc; a thermally insulated casing definingsaid first furnace chamber therein; one or more columns provided withinsaid first furnace chamber, each column comprising a plurality ofvertically orientated, vertically spaced, heat resistant tubes, whereinthe cross-sectional area of each tube is smaller than that of anadjacent, lower tube, and wherein the ends of adjacent tubes arearranged so as to provide an annular space therebetween; an inletthrough which a combustible charge is fed into the uppermost tube; anoutlet from which reacted charge is removed from the lowermost tube; anda fluid conduit for conveying combustible volatiles evolved by heatingsaid charge to a gas burning means for combustion, to thereby provideheat to said first furnace chamber.
 16. Apparatus according to claim 15,further comprising a third arrangement configured to control said zincvapour to substantially prevent or minimize its prematurerecondensation.
 17. Apparatus according to claim 16, wherein said thirdarrangement includes a further arrangement associated with said firstfurnace chamber whereby said zinc vapour is driven away from saidreduction zone.
 18. Apparatus according to claim 17, wherein said gasflow in said reduction zone is provided generally downwardly and saidzinc vapour is driven by said gas flow laterally for the reduction zone.19. Apparatus according to claim 15, wherein said first furnace chamberdefining a cooling zone comprises a zinc vapour condenser incommunication with an outlet from said reducing zone comprising a zincvapour collecting arrangement.
 20. Apparatus according to claim 19,wherein said condenser includes a zinc splash condenser.
 21. Apparatusaccording to claim 19, wherein said condenser includes a main condenserchamber having an inlet for receiving zinc vapour positioned above thebase of the main condenser chamber such that condensed zinc does notpass back into the furnace chamber.
 22. Apparatus according to claim 19,wherein said main condenser chamber surrounds a vapour conduit, the openend of which forms the vapour inlet, extending from said outlet from thereduction zone to a region of the condenser above its base. 23.Apparatus according to claim 19, wherein a vapour conduit extends in alateral direction from the furnace outlet into an upper region of thecondenser chamber.